Field of the Invention
The present invention relates to a regulator for fasteners, such as screws, bolts and rivets.
Description of the Related Art
When arranging a screw or a bolt, provided with a thread into a hole with a corresponding thread, a tool, such an electric wrench may be used. The screw or bolt has to be tightened to a predetermined torque in order to achieve a rigid joint. In mass production e.g. automobile production, when a large number of screws or bolts are to be applied, it is important that the screws or bolts are tightened to a predetermined torque in a short period of time.
In order to achieve the predetermined torque within a short time period a regulator is connected to the tool to regulate the tool.
A known regulator, which is used when tightening fasteners, is the PID regulator. Such a PID regulator gives an output signal u(t) according to:
                              u          ⁡                      (            t            )                          =                                            K              P                        *                          e              ⁡                              (                t                )                                              +                                    K              I                        ⁢                          ∫                                                e                  ⁡                                      (                    t                    )                                                  ⁢                dt                                              +                                    K              D                        ⁢                                          d                ⁢                                                                              dt                        ⁢                          e              ⁡                              (                t                )                                                                        (                  f          ⁢                                          ⁢          1                )            
e(t) is the deviation in the output signal and is received from a reaction in the system. A large value of KP leads to an increased rapidity of the regulator, but also to a decreased stability. The value of KI eliminates stationary errors in the output signal, but will decrease the stability if too large. Derivation, by using a proper value of KD will increase the stability. However, it is often connected to difficulties to find a proper value of KD, especially when the noise level of the output signal is high. The values of KP, KI and KD are chosen from general theory and are known for the skilled person.
However, one drawback using a PID regulator is that it is difficult to reach an acceptable torque within such a short period of time, since there has to be enough time left for compensation by KI and KD. As a result, the efficiency will decrease and therefore heat develops in the tool. Also, the load or strain on the power tool will increase due to large acceleration forces and variations in the current to the motor, which leads to low endurance of the tool and poor ergonomics for an operator handling the tool.